Sulphasalazine, sulphapyridine or 5-aminosalicylic acid--which is the active moiety in rheumatoid arthritis?

Sulphasalazine is cleaved into 5-aminosalicylic acid and sulphapyridine by colonic bacteria. The bulk of evidence favours sulphapyridine rather than 5-aminosalicylic acid as the active moiety (as well as the main producer of side-effects) though a therapeutic action from the 30% of sulphasalazine that is absorbed unaltered also cannot be excluded.